The present invention relates to linearity correction for the horizontal deflection system of a CRT monitor, and more particularly to a method of and a circuit for correcting the linearity of the horizontal deflection system of a CRT monitor, when image signals from imaging devices of different numbers of scanning lines are selectively displayed on the CRT monitor, by automatically correcting a distortion for display on the CRT monitor by using a function generator which generates distortion correcting functions corresponding to the numbers of scanning lines of the image signals applied to the CRT monitor.
Image signals having different numbers of scanning lines are required to be quickly displayed selectively on one CRT monitor in such medical imaging fields as CT (computerized tomography), US (ultrasonography), and DF (digital fluorography), for example, using a plurality of medical imaging diagnostic apparatus. A diagnostic system used in such a medical imaging field of art first displays image information supplied from the plural medical imaging diagnostic apparatus on a CRT monitor, and then photographs displayed images successively on a transported film through a photographic optical system comprising a lens, a shutter device, a mirror, and the like.
With this medical imaging technology, it is possible to obtain image information of an affected part and a surrounding region of a human body continuously and quickly according to the CT, US, DF, or the like. Therefore, the affected part in question and its surrounding region can clearly be comprehended by the doctor or the like for medical diagnosis. It is important therefore to obtain exact and fine image information to avoid a wrong diagnosis. To this end, a CRT monitor with as small distortions as possible is usually employed.
The horizontal linearity of such a CRT monitor, particularly a flat-face CRT monitor, is deteriorated by the following two factors or distortions:
One of the factors is a distortion as shown in FIGS. 1(a) and 1(b) of the accompanying drawings. This distortion is a combination of image compression at the center of the fluorescent screen of the CRT and image expansion at the opposite horizontal ends thereof due to the difference between the center of curvature of the CRT fluorescent screen and the deflection center of the electron beam. FIG. 1(a) schematically shows a displayed image when an input signal indicative of an equally spaced strips is applied to a CRT monitor with no distortion correction. Stripe-to-stripe intervals d.sub.2 at the opposite ends of the CRT screen are larger than stripe-to-stripe intervals d.sub.1 at the center of the CRT screen. FIG. 1(b) shows a characteristic curve of such a departure from horizontal linearity. The graph of FIG. 1(b) has a horizontal axis representative of the distance that the electron beam is deflected in the horizontal direction of the CRT screen and a vertical axis representative of a rate of change of the intervals with respect to the deflected distance.
The second distortion is illustrated in FIGS. 2(a) and 2(b). This distortion is caused by the resistance or the like of the horizontal deflection yoke, which prevents the horizontal deflection current from varying linearly but causes the horizontal deflection current to vary along an exponential saturated curve. As a result, the displayed image is elongated at the lefthand side of the screen, and is contracted at the righthand side of the screen. The displayed pattern shown in FIG. 2(a) and the graph shown in FIG. 2(b) can easily be understood by referring to the description of the displayed pattern shown in FIG. 1(a) and the graph shown in FIG. 1(b).
The first distortion can be corrected by adding a series resonance current of the horizontal deflection yoke of the CRT and a DC blocking capacitor (S correction capacitor) to a horizontal scanning sawtooth current for imparting an S-shaped waveform to the deflection current. It is possible to vary the resonance impedance according to the screen surface by selecting the value of the S correction capacitor, as shown in FIG. 3(a).
The second distortion can be corrected by connecting a saturable reactor biased by a DC magnetic field to the horizontal deflection yoke, the inductance of the reactor being variable according to the delection current flowing through the reactor. By employing a plurality of saturable reactors, the distortion can be corrected appropriately by varying a combined impedance composed of a lefthand inductance and a righthand inductance, as shown in FIG. 3(b).
FIG. 4 shows a conventional circuit for correcting the above two distortions. A horizontal deflection yoke 4 for horizontally deflecting an electron beam in a CRT 2 is connected in series to an S correction capacitor 10 and linearity correcting coils 6b, 8b of saturable reactors the inductances of which can be varied dependent on the current through magnets 6a, 8a, respectively. The horizontal linearity correcting circuit composed of the capacitor 10 and the saturable reactors for correcting the first and second distortions is driven by a horizontal output circuit 22 comprising a horizontal driver transformer 12, a horizontal output transistor 14, a damper diode 16, a resonant capacitor 18, and a choke coil 20 coupled to a power supply +V.sub.BB, for horizontally deflecting the electron beam and correcting the first distortion with S correction and the second exponential distortion.
The conventional horizontal linearity correcting circuit described above can only process a video signal having a particular number of scanning lines (horizontal frequency).
It is now assumed that a CRT monitor has 525 scanning lines (the horizontal frequency is about 15 KHz) and the power supply voltage is +V.sub.BB =15 V, and the S correction capacitor 10 and the linearity correcting coils 6b, 8b are adjusted for an optimum linearity correction constant. However, if a video signal of 1125 scanning lines (the horizontal frequency is about 33 KHz) is applied to this CRT monitor, then the image could not accurately be reproduced since the horizontal amplitude would be reduced. In this case, in order for the deflection width of the CRT screen, i.e., the current amplitude to remain the same for such a signal of a different number of scanning lines, as shown in FIG. 5(a) and 5(b), it is necessary that the power supply voltage +V.sub.BB be approximately doubled to 15.times.33 KHz/15 KHz=33 V. As a consequence, since the impedance of each component is varied according to the change of the horizontal frequency from 15 KHz to 33 KHz, the horizontal linearity cannot be well maintained unless the S correction capacitor 10 and the linearity correcting coils 6b, 8b are readjusted to match the varied horizontal frequency.
The loss of the time required for such readjustment is highly disadvantageous in the medical imaging diagnostic apparatus which is required to quickly display images on a CRT monitor in response to video signals applied from a plurality of imaging devices.